A ground power supply line for electric traction vehicles is provided with two conductors carried by a series of tiles aligned to one another; each tile has a conductive plate and a lower supporting structure, which is made of insulating material and accommodates electric connectors, normally open switches, and a control and command unit, which switches the switches for selectively electrically connecting the conductive plate to the conductors in response to a signal deriving from an electric traction vehicle passing over said external conductive plate; the upper surface of the conductive plates is flushed with the remaining part of a road surface.